The present invention relates to high transmittance alumina arc tubes for use in electric lamps. It finds particular application in conjunction with ceramic metal halide arc tubes, and will be described with particular reference thereto. It should be appreciated, however, that the invention is also applicable to other lamp envelopes and shrouds for lamps where high transmittance is desired.
Metal halide lamps have conventionally been constructed of a fused silica (quartz) arc tube containing a fill of a light-emitting metal, such as sodium, commonly in the form of the halide, and optionally mercury. The lifetime of such lamps is often limited by the loss of the metal portion of the metal halide fill during lamp operation due to metal ion diffusion, or reaction of the metal halide with the fused silica arc tube, and a corresponding build-up of free halogen in the arc tube.
Recently, ceramic metal halide lamps having polycrystalline alumina arc tubes have been developed which provide advantages over quartz arc tubes. U.S. Pat. Nos. 5,424,609; 5,698,948; and 5,751,111 provide examples of such arc tubes. Ceramic alumina arc tubes are less permeable to sodium ions than quartz and thus retain the metal within the lamp. They are also able to withstand much higher operating temperatures than quartz arc tubes. While quartz arc tubes are limited to operating temperatures of around 900-1000xc2x0 C., due to reaction of the halide fill with the glass, ceramic alumina arc tubes are capable of withstanding operating temperatures of 1100 to 1200xc2x0 C., or higher. The higher operating temperatures provide better color rendering and higher lamp efficiency.
Alumina arc tubes are generally constructed of a number of separate parts. The parts are extruded or die pressed from a ceramic powder mixed with an organic binder. European patent Application No. 0 587 238 A1, for example, discloses a ceramic discharge tube of translucent aluminum oxide. Typically, the parts are tacked together with an adhesive and then sintered to form gas-tight monolithic joints between the components.
Another potential arc tube material for metal halide lamps is sapphire. Sapphire arc tubes have been found to provide improved lamp performances over alumina arc tubes due to increased transmission levels. However, such lamps are expensive due to the cost of manufacturing the monocrystalline sapphire material. There are also problems in sealing of the lamps to prevent loss of the fill material.
Improvements in the transmittance of polycrystalline alumina arc tubes have been found when the arc tubes are chemically polished with an alkali metal borate composition. U.S. Pat. Nos. 4,033,743, and 4,633,137 to Scott, et al. disclose a method of contacting an arc tube body with a molten inorganic borate flux which preferentially dissolves a surface layer of alumina grains. The process does not, however, provide arc tubes with transmittances comparable to sapphire because of microscopic discontinuities, or porous regions, in the arc tube surface. The discontinuities remain, even after polishing, reducing the transmittance of the arc tube.
The present invention provides for an improved ceramic body, such as a metal halide arc tube and method of preparation, which has optical performance characteristics approaching those of sapphire.
In an exemplary embodiment of the present invention, a method of providing a translucent ceramic body with increased in-line optical transmission is provided. The method includes densifying a ceramic body to form a substantially translucent ceramic body. The densifying process includes heating the ceramic body under a pressure of at least 350 kg/sq.cm. The method further includes physically contacting a major surface of the substantially translucent ceramic body with a molten inorganic flux at elevated temperatures and for a time period sufficient to improve transmittance of the ceramic body. The flux includes an alkali metal borate capable of dissolving the ceramic.
In another exemplary embodiment, an optically transparent densified, sintered polycrystalline ceramic body is provided. The body has a major surface which has been treated with a process which includes heating a ceramic body in an inert atmosphere a pressure of at least 350 kg/sq.cm for a sufficient time to form a substantially translucent polycrystalline ceramic body. The process further includes physically contacting a major surface of the substantially translucent ceramic body with a molten inorganic flux which includes an alkali metal borate capable of dissolving the ceramic at elevated temperatures and for a time period sufficient to improve light transmittance by the ceramic body.
In another exemplary embodiment, a high intensity electric discharge lamp is provided. The lamp includes a discharge vessel which defines a chamber. The discharge vessel is constructed from a polycrystalline material which has been densified by applying sufficient pressure and temperature to reduce pores in the vessel and polished by physically contacting a major surface of the substantially translucent vessel with a molten inorganic flux at an elevated temperature and for a time period sufficient to reduce unevenness in the major surface. The lamp further includes electrodes sealed into ends of the chamber and a fill sealed within the chamber. The fill includes a ionizable medium for initiating and sustaining a discharge.
One advantage of the present invention is that it enables an alumina arc tube with high transmittance to be formed.
Another advantage of the present invention which derives from the ability of the arc tube to transmit light with minimal scattering from the smooth surface and allows for lamps formed from the material to provide a more point source illumination.
Still further advantages of the present invention will become apparent to those of ordinary skill in the art upon reading and understanding the following detailed description of the preferred embodiments.